Proton NMR Study of Oxo-Bridged Dimanganese(III) Complexes:  Solution State Structures and an Isotropic Shift/Magnetic Exchange Correlation

• Published in: Inorganic chemistry 1998-07, Vol.37 (15), p.3714-3718
• Main Authors: Wright, David W, Mok, Henry J, Dubé, Christopher E, Armstrong, William H
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic960646l

The 1H NMR spectra of a series of manganese−oxo aggregates have been examined, and a characteristic signature was found for each complex. For the dimanganese(III,III) complexes [Mn2O(OAc)2(HB(pz)3)2], [Mn2O(OAc)2(tacn)2]2+, [Mn2O(OAc)2(H2O)2(bpy)2]2+, and [Mn2O(OAc)2(bpta)2]2+ (HB(pz)3 = hydrotris(pyrazol-1-yl)borate; tacn = 1,4,7-triazacyclononane; bpy = 2,2‘-bipyridine, and bpta = N,N-bis(2-pyridylmethyl)-tert-butylamine), the 1H NMR spectra reveal a resonance associated with acetate, found downfield between 58 and 80 ppm, and a generally well resolved set of terminal ligand resonances which can be divided into two classes:  those resonances associated with pyridyl or pyrazolyl ring protons and those of methylene groups. A number of the pyridine ring resonances have been unambiguously assigned by the examination of methyl-substituted derivatives. Data for these derivatives also support a coordination geometry-dependent pathway for spin delocalization. Moreover, interpretation of the 1H NMR spectra leads to the conclusion that the solution-state structures of all members of the series are the same as the reported solid-state structures. A strong linear correlation between the magnetic coupling constant (J) and the isotropic shift of the acetate resonance was observed within this series of {Mn2O(OAc)2}2+ core complexes. Furthermore, comparisons of the acetate proton isotropic shift ratio (ΔH Mn/ΔH Fe) to the ratio of the squared effective magnetic moments μeff 2(Mn)/ μeff 2(Fe) for complexes with the {M2O(OAc)2}2+ core (where M = Mn3+ or Fe3+) revealed excellent agreement (within 10%) between these two quantities.